How can I get the index of a custom property?
Lets dissect what your code does:

key = bpy.context.active_object.keys()[2]
# key contains the name '02_back'

char = key[2]
# gets the third character which is '_'

bpy.context.active_object.get(char)
# which throws an error as key '_' does not exist


It should be

bpy.context.active_object.get(bpy.context.active_object.keys()[2])[2]


Let's dissect this

key = bpy.context.active_object.keys()[2]
# key contains the name '02_back'

prop = bpy.context.active_object.get(key)
# gets the property with the name '02_back'

value = prop[2]
# gets the third item of the property `1.2`